As described in U.S. Pat. No. 4,096,743, shaft torque is commonly measured by determining the amount of twist in the shaft as a result of the torque exerted on the shaft. Conventionally, the twist is measured by means of a transducer attached at axially spaced-apart portions of the shaft such that the twisting stress on the transducer produces an electrical signal respresentative of shaft torque. In order to supply electrical power to the transducer and any electrical circuits mounted on the shaft, a slip ring and brush assembly is utilized in the prior art. The slip ring and brush assembly is mounted on the rotating shaft and transmits the electrical shaft torque signal produced by a transducer to an electrical stationary circuit which is calibrated to measure the shaft's torque. The disadvantage of prior art shaft torque measuring systems utilizing slip and brush assemblies is that the electrical contact between the brushes and the rings can deviate from an ideal connection due to variations in electrical resistance of the rings and brushes. To minimize the electrical resistance variations, it is necessary to use an expensive material for the slip rings and brushes. For this purpose, coin silver and silver-gold alloys are generally used for slip ring material. High quality brushes are made of alloys containing a mixture of gold, palladium, platinum and silver. These materials minimize the possibility of false signals resulting from tarnish embedment of foreign matter and variations in constriction resistance of the contact area.
This invention avoids the necessity for employing precious metals in the slip ring and brush assembly by converting the transducer voltage signals to current signals which are unaffected by changes in line and contact resistance.